This invention relates generally to tube fittings and particularly to fittings for forming a strong fluid-tight joint between straight ended tubes and the fitting body.
Many types of tube fittings have heretofore been utilized for making a strong, fluid-tight connection between the fitting and the end of a tube of circular cross section. The present invention relates to an improvement in one class of these fittings generally known in the art as "bite-type" or flareless fittings. Such flareless fittings generally include a body member surrounding the end of the tube, a nut which also surrounds the tube and threadingly engages the body member, and one or more ferrules placed in axial compression between the nut and the body member and forced against the outer wall of the tube in a biting fashion. The ferrules are designed to create both a fluid-tight seal and a strong mechanical grip. An example of such a tube coupling is found in U.S. Pat. No. 2,484,815 to Crawford. The coupling shown in the Crawford patent includes two ferrules, one ferrule primarily being utilized to obtain a good fluid-tight seal and a second ferrule primarily employed to obtain a strong mechanical grip between the coupling and the tube. Although such two ferrule type fittings have generally worked well, there have been some problems and shortcomings associated with their use. Fittings of this general type are nearly always assembled or made up under field conditions by any one of a large number of technicians, any one of whom may have a relatively limited experience with a particular type of fitting. Thus, as one increases the number of parts in the fitting, as is the case in the two ferrule construction, one greatly increases the probability that the fitting will be improperly assembled. For example, in the type of fitting shown in the above-mentioned Crawford patent, it is possible for one to assemble the ferrules in as many as eight different ways. However, there is only one proper sequence and direction if adequate gripping and sealing is to be obtained.
To overcome the problem of improper assembly, which exists even with single ferrule designs, a number of couplings have been designed in which a sleeve is attached to the nut and fractures during assembly to create both a seal and a mechanical joint. Examples of such couplings appear in U.S. Pat. No. 1,889,778 to Dobrick, U.S. Pat. No. 3,743,324 to Schwarz, et al. and U.S. Pat. No. 4,022,497 to Katsakis. These couplings are generally effective in creating a reasonably strong good sealing joint. However, since couplings of this type result in the utilization of only a single ferrule for both sealing and mechanical strength, they generally do not perform as well as the aforementioned double ferrule design.
Another problem associated with fittings of this general type is the requirement that the biting edges of the ferrules be harder than the tubing to obtain a strong reliable mechanical connection. In the double ferrule type construction shown in the aforementioned Crawford patent, this requirement would particularly apply to the ferrule adjacent the nut. In certain chemical processes, it may be undesirable to have a fitting made of more than one material. For example, if corrosion resistance is required, it may be that the fitting must be made entirely of austenitic stainless steel and be suitable for use with tubing which is made of the same material. Austenitic stainless steel cannot be hardened by heat treatment, and suffers from a reduction in its corrosion resistance if it is hardened by known case-hardening methods; this means that there is no readily available technique for assuring that the biting edges of the ferrule are harder than the tubing in order to assure a strong connection.
An additional shortcoming of the double ferrule construction is that the cost of manufacturing additional parts increases the overall manufacturing costs of the coupling.